3 Review of Fission235U will undergo spontaneous fission if a neutron happens by, resulting in:two sizable nuclear fragments flying outa few extra neutronsgamma rays from excited states of daughter nucleienergetic electrons from beta-decay of daughtersThe net result: lots of banging aroundgenerates heat locally (kinetic energy of tiny particles)for every gram of 235U, get 65 billion Joules, or about 16 million Caloriescompare to gasoline at roughly 10 Calories per grama tank of gas could be replaced by a 1-mm pellet of 235U!!

4 Enrichment Natural uranium is 99.27% 238U, and only 0.72% 235U238U is not fissile, and absorbs wandering neutronsIn order for nuclear reaction to self-sustain, must enrich fraction of 235U to 3–5%interestingly, it was so 3 billion years agonow probability of wandering neutron hitting 235U is sufficiently high to keep reaction crawling forwardEnrichment is hard to do: a huge technical roadblock to nuclear ambitions

9 iClicker QuestionA neutron decays. It has no electric charge. If a proton (positively charged) is left behind, what other particle must come out if the net charge is conserved?A No other particles are needed.B A negatively charged particle must emerge as well.C A positively charged particle must emerge as well.D Another charge will come out, but it could be either positively charged or negatively charged.E Neutrons cannot exist individually.

10 iClicker QuestionA neutron decays. It has no electric charge. If a proton (positively charged) is left behind, what other particle must come out if the net charge is conserved?A No other particles are needed.B A negatively charged particle must emerge as well.C A positively charged particle must emerge as well.D Another charge will come out, but it could be either positively charged or negatively charged.E Neutrons cannot exist individually.

28 Nuclear power around the globe17% of world’s electricity from nuclear powerU.S. about 20% (2nd largest source)431 nuclear plants in 31 countries103 of them in the U.S.Built none since 1970sU.S. firms have exported nukes.Push from Bush/Obama for new nukes.

39 Radioactivity Basics UnitsRadioactivity – The spontaneous nuclear transformation of an unstable atom that often results in the release of radiation, also referred to as disintegration or decay.UnitsCurie (Ci) the activity in one standard gram of Radium = 3.7 x 1010 disintegrations per secondBecquerel (Bq) 1 disintegration per second – International Units (SI)

40 Radioactivity BasicsRadiation – Energy in transit in the form of electromagnetic waves (gamma-γ or x-ray), or high speed particles ( alpha-α, beta-β, neutron-η, etc.)Ionizing Radiation – Radiation with sufficient energy to remove electrons during interaction with an atom, causing it to become charged or ionized.Can be produced by radioactive decay or by accelerating charged particles across an electric potential.

41 Radioactivity BasicsRoentgen R the unit of exposure to Ionizing Radiation. The amount of γ or x-ray radiation required to produce 1.0 electrostatic unit of charge in 1.0 cubic centimeter of dry air.Rad the unit of absorbed dose. Equal to 100 ergs per gram of any material from any radiation.SI unit = Gray1 Gray = 100 radsREM the unit of absorbed dose equivalent. The energy absorbed by the body based on the damaging effect for the type of radiation.REM =Rad x Quality FactorSI unit = Sievert Sv = 100 Rem

42 iClicker Question Which of the following describes the Roentgen?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

43 iClicker Question Which of the following describes the Roentgen?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

44 iClicker Question Which of the following describes the RAD?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

45 iClicker Question Which of the following describes the RAD?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

46 iClicker Question Which of the following describes the REM?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

47 iClicker Question Which of the following describes the REM?A the unit of absorbed dose equivalent.B the unit of absorbed dose.C the unit of exposure to ionizing radiationD all of the aboveE none of the above

48 ALARAA philosophy, necessary to maintain personnel exposure or the release of radioactivity to the environment well below applicable limits by means of a good radiation protection plan, through education, administrative controls and safe lab practices.As Low As Reasonably Achievable

49 ALARA Principles DistanceInverse Square Law – radiation intensity is inversely proportional to the square of the distance from the sourceUse remote handling tools, or work at arms lengthMaximize distance from source of radiation

50 ALARA Principles ShieldingAny material between a source of radiation and personnel will attenuate some of the energy, and reduce exposureSelect proper shielding material for type of radiation, use less dense material for β’s, to minimize Bremsstrahlung (braking) radiation

51 Background RadiationBelow are estimates of natural and man-made background radiation at sea level at middle latitudes. The total averages 400 – 500 mREM/yrNatural Sources (300 mREM): "Natural" background radiation consists of radiation from cosmic radiation, terrestrial radiation, internal radionuclides, and inhaled radon.Occupational Sources (0.9 mREM): According to NCRP Report No. 93, the average dose for workers that were actually exposed to radiation in 1980 was approximately 230 mREM.The Nuclear Fuel Cycle (0.05 mREM): Each step in the nuclear fuel cycle can produce radioactive effluents in the air or water.Consumer Products (5-13 mREM): The estimated annual dose from some commonly-used consumer products such as cigarettes (1.5 pack/day, 8,000 mREM) and smoke detectors (1 mREM) contribute to total annual dose.Miscellaneous Environmental Sources (0.6 mREM): A few environmental sources of background radiation are not included in the above categories.Medical Sources (53 mREM): The two contributors to the radiation dose from medical sources are diagnostic x-rays and nuclear medicine. Of the estimated 53 mREM dose received annually, approximately 39 mREM comes from diagnostic x-rays.

55 Long-term Effects of Radiation ExposureLong-term effects are usually related to increased risk of cancer, summarized in the table below:DiseaseAdditional Cases per 100,000 (with one-time 10 REM dose) *Adult leukemia95Cancer of digestive system230Cancer of respiratory system170* Source: Biological Effects of Ionizing Radiation V (BEIR V) CommitteeRadiation-induced life shortening (supported by animal experiments) suggests accelerated aging may result in the loss of a few days of life as a result of each REM of exposureGenetic Effects of radiation fall into two general categoriesEffect on individuals: Can change DNA and create mutation but long term effects not well understood. Biological repair mechanisms may reduce importance.Effect of offspring: Exposure to a fetus in utero can have profound effects on developing organs resulting in severe birth defects. For this reason pregnant women should avoid any non-background exposures

56 Bioeffects on Surface tissuesBecause of the low energy (~8 keV for Cu) of analytical x-rays, most energy will be absorbed by skin or other exposed tissueThe threshold of skin damage is usually around 300 R resulting in reddening of the skin (erythema)Longer exposures can produce more intense erythema (i.e., “sunburn”) and temporary hair lossEye tissue is particularly sensitive – if working where diffracted beams could be present, eye protection should be worn

60 Nuclear Fission ReactorsNuclear fission is used simply as a heat source to run a heat engineBy controlling the chain reaction, can maintain hot source for periods greater than a yearHeat is used to boil waterSteam turns a turbine, which turns a generatorEfficiency limited by familiar Carnot efficiency: = (Th - Tc)/Th (about 30–40%, typically)

62 The Core of the Reactor not shown are the control rods that absorbneutrons andthereby keep theprocess fromrunning away

63 Fuel PackagingWant to be able to surround uranium with fluid to carry away heatlots of surface area is goodAlso need to slow down neutronswater is good for thisSo uranium is packaged in long rods, bundled into assembliesRods contain uranium enriched to ~3% 235UNeed roughly 100 tons per year for a 1 GW plantUranium stays in three years, 1/3 cycled yearly

64 Control Rod Action Basic Conceptneed exactly one excess neutron per fission event to find another 235UInserting a neutron absorber into the core removes neutrons from the poolPulling out rod makes more neutrons availableEmergency procedure is to drop all control rods at once

65 California Nuclear Plant at San Onofre10 miles south of San ClementeEasily visible from I-52 reactors brought online in 1983, 1984older decommissioned reactor retired in 1992 after 25 years of service1.1 GW eachPWR (Pressurized Water Reactor) typeNo cooling towersthe ocean is used

67 Sidebar Regarding Nuclear BombsSince neutrons initiate fission, and each fission creates more neutrons, there is potential for a chain reactionHave to have enough fissile material around to intercept liberated neutronsCritical mass for 235U is about 15 kgfor 239Pu it’s about 5 kgneed highly enriched (about 90% 235U for uranium bomb)Bomb is relatively simpleseparate two sub-critical masses and just put them next to each other when you want them to explode!difficulty is in enriching natural uranium to mostly 235U

73 The finite uranium resourceUranium cost is about $23/kgabout 1% of cost of nuclear powermore expensive to get as we deplete the easy spotsEstimated 3 million tons available at cost less than $230/kgNeed 200 tons per GW-yrNow have 100 GW of nuclear power generationabout GW each3 million tons will last 150 years at present rateonly 30 years if nuclear replaced all electricity production

74 Breeder ReactorsThe finite resource problem goes away under a breeder reactor programNeutrons can attach to the non-fissile 238U to become 239Ubeta-decays into 239Np with half-life of 24 minutes239Np beta-decays into 239Pu with half-life of 2.4 daysnow have another fission-able nuclideabout 1/3 of energy in normal reactors ends up coming from 239PuReactors can be designed to “breed” 239Pu in a better-than-break-even way

76 Breeders, continuedCould use breeders to convert all available 238U into 239Puall the while getting electrical power outNow 30 year resource is 140 times as much (not restricted to 0.7% of natural uranium), or 4200 yrTechnological hurdle: need liquid sodium or other molten metal to be the coolantbut four are running in the worldEnough 239Pu falling into the wrong hands spells:BOOM!!

77 Reactor Risks Once a vigorous program in the U.S.in France80% of electricity is nuclearNo new orders for reactors in U.S. since late 70’saftershock of Three-Mile IslandReactor failure modes:criticality accident: runaway chain reactionmeltdownloss of cooling: not runaway, but overheats meltdownsteam or chemical explosions are not ruled out meltdownN.B. reactors are incapable of nuclear explosion

78 Risk Assessment Extensive studies by agencies like the NRC1975 report concluded that:loss-of-cooling probability was 1/2000 per reactor yearsignificant release of radioactivity 1/1,000,000 per RYchance of killing 100 people in an accident about the same as killing 100 people by a falling meteor1990 NRC report accounts for external disasters (fire, earthquake, etc.)large release probability 1/250,000 per RY109 reactors, each 30 year lifetime  1% chance

82 Health around TMIIn 1979, hundreds of people reported nausea, vomiting, hair loss, and skin rashes. Many pets were reported dead or showed signs of radiationLung cancer, and leukemia rates increased 2 to 10 times in areas within 10 miles downwindFarmers received severe monetary losses due to deformities in livestock and crops after the disaster that are still occurring today.

85 The Chernobyl DisasterDisregard of safety standards plus unstable design led to disasterChernobyl was a boiling-water, graphite-moderated designunlike any in the USAused for 239Pu weapons productionfrequent exchange of rods to harvest Pu meant lack of containment vessel like the ones in USApositive-feedback effectIt gets too hot, it runs hotterrunaway possibleonce runaway, control rods ineffective

86 Chernobyl, continuedOn April 25, 1986, operators decided to do an “experiment” as the reactor was powering down for routine maintenancedisabled emergency cooling system!!!withdrew control rods completely!!!powered off cooling pumps!!!reactor went out of control, caused steam explosion that ripped open the reactormany fires, exposed core, major radioactive release

87 Chernobyl after-effectsTotal of 100 million people exposed (135,000 lived within 30 km) to radioactivity much above natural levelsExpect from 25,000 to 50,000 cancer deaths as a resultcompared to 20 million total worldwide from other causes20,000,000 becomes 20,050,000 (hard to notice……unless you’re one of those 50,00031 died from acute radiation exposure at site200 got acute radiation sickness

90 Radiation and Health Health effects as a result of radiation exposure:-increased likelihood of cancer-birth defects including long limbs, braindamage, conjoined stillborn twins-reduced immunity-genetic damage

92 iClicker QuestionConsider all of the people throughout history who have been exposed to man-made nuclear radiation, such as Hiroshima and Nagasaki, Chernobyl, Three Mile Island, nuclear bomb tests, accidental spills, etc.Which number most nearly approximates how many children conceived and born later to these people suffered genetic damage due to a parent’s exposure, excluding exposure during pregnancy?A. ~ millionsB. ~ thousandsC. ~ hundredsD. ~ zero

93 iClicker QuestionConsider all of the people throughout history who have been exposed to man-made nuclear radiation, such as Hiroshima and Nagasaki, Chernobyl, Three Mile Island, nuclear bomb tests, accidental spills, etc.Which number most nearly approximates how many children conceived and born later to these people suffered genetic damage due to a parent’s exposure, excluding exposure during pregnancy?A. ~ millionsB. ~ thousandsC. ~ hundredsD. ~ zero

94 Nuclear ProliferationThe presence of nuclear reactors means there will be plutonium in the worldand enriched uraniumIf the world goes to large-scale nuclear power production (especially breeder programs), it will be easy to divert Pu into nefarious purposesBut other techniques for enriching uranium may become easy/economicaland therefore the terrorist’s top choiceShould the U.S. abandon nuclear energy for this reason?perhaps a bigger concern is all the weapons-grade Pu already stockpiled in the U.S. and former U.S.S.R.

96 Nuclear WasteEach reactor has storage pool, meant as temporary holding placeoriginally thought to be 150 days40 years and countingVariety of radioactive products, with a wide range of half-lives1GW plant waste is 70 MCi after one year; 14 MCi after 10 years; 1.4 MCi after 100 years; MCi after 100,000 years1 Ci (Curie) is 37 billion radioactive decays per second

97 Storage Solutions No failsafe storage solution yet developedEPA demands less than 1000 premature cancer deaths over 10,000 years!!hard to design and account for all contingenciesUSA proposed site at Yucca Mountain, NVGood and bad choicegeologically: cracks and questionable stability

98 Burial Issues Radioactive emissions themselves are not radioactivejust light, electrons/positrons and helium nucleibut they are ionizing: they rip apart atoms/molecules they encounterAbsorb emissions in concrete/earth and no effect on biologyso burial is good solutionProblem is the patience of timehalf lives can be longgeography, water table changesnature always outlasts human structuresimagine building something to last 10,000 years!!

105 iClicker QuestionSuppose that all of the electrical energy for the world for the next 500 years were obtained from nuclear reactors. Further suppose that all of the nuclear waste from these reactors were dissolved and spread uniformly throughout the oceans of the world.Which statement is true:A. The oceans would be a vast wasteland, unable to support life.B. Much death and damage to ocean life would be caused.C. Any effect would be so small that it would be virtually impossible to see.

106 iClicker QuestionSuppose that all of the electrical energy for the world for the next 500 years were obtained from nuclear reactors. Further suppose that all of the nuclear waste from these reactors were dissolved and spread uniformly throughout the oceans of the world.Which statement is true:A. The oceans would be a vast wasteland, unable to support life.B. Much death and damage to ocean life would be caused.C. Any effect would be so small that it would be virtually impossible to see.

107 Fusion: The big nuclear hopeRather than rip nuclei apart, how about putting them together?alpha (4He)Iron is most tightly bound nucleusCan take loosely bound light nuclei and build them into more tightly bound nuclei, releasing energyHuge gain in energy going from protons (1H) to helium (4He).It’s how our sun gets its energyMuch higher energy content than fissiontritiumdueteriumproton

108 Thermonuclear Fusion in the SunSun is 16 million degrees Celsius in centerEnough energy to ram protons together (despite mutual repulsion) and make deuterium, then heliumReaction per mole ~20 million times more energetic than chemical reactions, in general4 protons:mass = 4.029neutrinos, photons(gamma rays)4He nucleus:mass =

109 E=mc2 balance sheets Helium nucleus is lighter than the four protons!Mass difference is – = a.m.u.0.7% of mass disappears, transforming to energy1 a.m.u. (atomic mass unit) is 10-27 kgdifference of 4.5810-29 kgmultiply by c2 to get 4.1210-12 J1 mole (6.0221023 particles) of protons  2.51012 Jtypical chemical reactions are 100–200 kJ/molenuclear fusion is ~20 million times more potent stuff!works out to 150 million Calories per gramcompare to 16 million Cal/g uranium, 10 Cal/g gasoline

112 Deuterium everywhere Natural hydrogen is 0.0115% deuteriumLots of hydrogen in sea water (H2O)Total U.S. energy budget (100 QBtu = 1020 J per year) covered by sea water contained in cubic volume 170 meters on a sidecorresponds to 0.15 cubic meters per secondabout 1,000 showers at two gallons per minuteabout one-millionth of rainfall amount on U.S.~4 gallons per person per year

113 Tritium Nowhere Tritium is unstable, with half-life of 12.32 yearsthus none naturally availableCan make it by bombarding 6Li with neutronsextra n in D-T reaction can be used for this, if reaction core is surrounded by “lithium blanket”Lithium on land in U.S. would limit D-T to a hundred years or somaybe a few thousand if we get lithium from oceanD-D reaction requires higher temperature, but could be sustained for many millennia

114 By-products? Not like radioactive fission productsBuilding stable nuclei (like 4He)Tritium is only radioactive substanceenergy is low, half-life short: not much worry hereExtra neutrons can tag onto local metal nuclei (in surrounding structure) and become radioactivebut this is a small effect, especially compared to fission

116 Fusion Successes?Fusion has been accomplished in labs, in big plasma machines called Tokamaksgot ~6 MW out of Princeton Tokamak in 1993but put ~12 MW into it to sustain reactionHydrogen bomb also employs fusionfission bomb (e.g., 239Pu) used to generate extreme temperatures and pressures necessary for fusionLiD (lithium-deuteride) placed in bombfission neutrons convert lithium to tritiumtritium fuses with deuterium

118 Passive Radioactive DecayRadioisotope Thermoelectric GeneratorObtains power from passive radioactive decaysUtilized in satellites and space probesSeebeck/Peltier effectJunction of two dissimilar metals at different temperatures create a currentFuelLong half life, low shielding (beta decay)Plutonium 238 most common

119 The Peltier/Seebeck EffectBy Jacob McKenzie, Ty Nowotny, Colin NeunuebelDiscovered by Thomas Johann Seebeck in 1821.He accidentally found that a voltage existed between two ends of a metal bar when a temperature gradient existed within the bar.

120 where α is the Seebeck coefficient of the coupleThe Seebeck EffectA temperature difference causes diffusion of electrons from the hot side to the cold side of a conductor.The motion of electrons creates an electrical current.The voltage is proportional to the temperature difference as governed by: V=α(Th-Tc)where α is the Seebeck coefficient of the couple

121 History of Peltier devicesThe Peltier effect is named after Jean Charles Peltier ( ) who first observed it in 1834.The Peltier effect had no practical use for over 100 years until dissimilar metal devices were replaced with semiconductor Peltiers which could produce much larger thermal gradients.Peltier Cooler - produce a temperature gradient that is proportional to an applied current

122 Peltier Effect With Dissimilar MetalsAt the junction of two dissimilar metals the energy level of conducting electrons is forced to increase or decrease.A decrease in the energy level emits thermal energy, while an increase will absorb thermal energy from its surroundings.The temperature gradient for dissimilar metals is very small.The figure of merit is a measure ofthermoelectric efficiency.

123 Sidebar: Semiconductor PeltierBismuth-Telluride n and p blocksAn electric current forces electrons in n type and holes in p type away from each other on the cold side and towards each other on the hot side.The holes and electrons pull thermal energy from where they are heading away from each other and deliver it to where they meet.

128 Wikipedia Reports on Disaster at Fukushima“An earthquake categorized as 9.0 on the moment magnitude scale occurred on 11 March 2011, at 14:46 Japan Standard Time (JST) off the northeast coast of Japan. On that day, reactor units 1, 2, and 3 were operating, but units 4, 5, and 6 had already been shut down for periodic inspection. When the earthquake was detected, units 1, 2 and 3 underwent an automatic shutdown (called scram).“After the reactors shut down, electricity generation stopped. Normally the plant could use the external electrical supply to power cooling and control systems, but the earthquake had caused major damage to the power grid. Emergency diesel generators started correctly but stopped abruptly at 15:41, ending all AC power supply to the reactors. The plant was protected by a sea wall, but tsunami water which followed after the earthquake topped this sea wall, flooding the low lying generator building…“After the failure of the diesels, emergency power for control systems was supplied by batteries that would last about eight hours. Batteries from other nuclear plants were sent to the site and mobile generators arrived within 13 hours, but work to connect portable generating equipment to power water pumps was still continuing as of 15:04 on 12 March. Generators would normally be connected through switching equipment in a basement area of the buildings, but this basement area had been flooded by the tsunami.”